1. Field of the Invention
This invention relates generally to an image registration system for registering two images from two separately located sensors and more particularly to an image registration system for registering an image from a forward-looking infrared navigation and target acquisition/tracking system on an aircraft and an image from a weapon system being delivered to a target by the aircraft.
2. Description of the Prior Art
Forward-looking Infrared (FLIR) systems are generally known, particularly, as they relate to their use in connection with military aircraft which are required to fly at low altitude and acquire targets at night for the purpose of delivering armament or ordnance against them.
A FLIR system can be used to perform a variety of functions. These range from all-weather navigation aids to active air-air search and track. One use of the FLIR system is for aiding delivery of infrared (IR) missile weapons systems to a designated target. Target acquisition can be performed using infrared sensors on the aircraft having a greater range than the missile's sensors. Registration of the aircraft image with the missile image would allow for automatic delivery of a the missile at maximum range from a heads up display (HUD). This is in contrast to the current system which requires the pilot to use a cockpit display to acquire targets on inferior missile imagery and command launch equipment.
In order for the FLIR system to control the missile, the images produced by the two systems must be registered. Using direction information from the FLIR gimbal subsystem, the system can slew the missile sensor to the same approximate target location. The precise location will not be known due to several factors. The first of these is the spatial difference in location of the FLIR system and missile sensors. On an aircraft in flight, a missile, mounted under the wing, will undergo constant movement with respect to the infrared sensor mounted on the fuselage resulting in inconsistent spatial separation. Related to this are problems with improper alignment of the missile during mounting which can result in a translation and rotation of the infrared sensor of the missile from its nominal position. Another problem which complicates the registration is the difference in optics and sensitivity between the two sensors. While the FLIR sensor typically has a linear mapping in all directions of all points on the sensor, a missile normally has "fisheye" optics which result in a nonlinear mapping with higher resolution in the center. Further, the registration must be highly precise. At a distance of two miles, a 0.1 degree aiming difference results in a lateral difference of over eighteen (18) feet, the size of a typical target.
The image registration must compare the two images to determine the mapping from one to the other. A conventional method of analyzing the images would be to compare the two images on a pixel by pixel basis. This however would require a large number of operations. Furthermore, a 592.times.480 FLIR image frame has a total of 284K pixels which would have to be individually mapped and compared with the missile image pixels. Instead of comparing each individual pixel, the method and apparatus the present application compare the image "hot spots". The image "hot spots" are provided by a target detector such as a double window filter. Given that the registration is to be performed on the hot spots, the problem becomes one of determining the best mapping from the hot spot coordinates of one image to the corresponding coordinates of the other. The method and apparatus for registering two images is described herein. It should be noted that the method and apparatus of the present application can be used for registration of any images, not just those from the system to be described.